Flash memory has developed into a popular source of non-volatile, electrically erasable memory in a wide range of digital applications. Flash memory devices typically use a one-transistor memory cell which allows for high memory densities, high reliability and low power consumption. These characteristics have made flash memory very popular for low power applications, such as battery-backed or embedded memory circuits.
A crucial circuit in the operation of flash memory is the precision voltage reference circuit. In a precision voltage reference circuit, a differential amplifier is typically used to sense a difference in current between two flash cells. Flash cells can also be used in other comparator or sense amplifier circuits to provide reference voltages for the inputs of differential amplifiers.
The use of flash memory in low voltage applications has encouraged the production of such devices through production processes that are capable of producing small-scale devices that feature high-speed and low power consumption. However, as process technology shrinks the feature sizes of transistors, integrated circuit operating voltages must be reduced every process generation to limit the tolerant electric fields across the terminals of the transistors. For example, in certain semiconductor manufacturing processes, voltages in excess of 2.0V across the gate-drain junctions of the transistors pose a potential for gate oxide breakdown, thus negatively impacting the reliability and quality of the devices produced through these processes. Although transistors produced with the latest process technology often feature faster performance and lower power supply requirements, certain devices produced by a particular semiconductor production process may exhibit different slew rate and gain characteristics from other devices produced by that process.
When used in low-voltage applications, such variations in amplifier characteristics among devices in a circuit may become quite critical. This is due to the fact that low-voltage comparators and differential amplifiers are often limited to relatively small input voltage ranges because the rail-to-rail common mode input range is limited by a low supply voltage and the fixed threshold voltage of the transistor devices comprising the differential amplifier.
Differential amplifier circuits produced by present semiconductor production processes are produced with a fixed bias current source. The amount of bias (or tail) current used to drive a differential amplifier affects the slew rate (speed) of the amplifier, as well as the differential gain and common-mode range of the amplifier. However, if the bias current source is fixed, these characteristics cannot easily be changed to alter the performance of a differential amplifier, or to compensate for variations in the characteristics of devices made from a particular production process.